The inkjet printing method is one of the recoding methods in which ink droplets are directly ejected from microscopic nozzles onto a recording medium and are fixed thereon, thereby forming characters and images. The inkjet printing method has been now widely used because of its advantages that the inkjet machines are less noisy and well operable, the colorization is easy, and plain paper is usable as the recording medium. As compared with the electrophotographic printing method widely spreading in office use and quick printing market ahead, the inkjet printing method is an energy-saving method because the heat fixing is not needed. With this additional advantage, the inkjet printing method is getting attention in the market. In particular, an inkjet ink is capable of providing printed matters with a high durability comparable to that of the electrophotographic printing method has been recently developed by using a pigment as the colorant.
However, as compared with the electrophotographic printing method of forming color images on plain paper, the inkjet printing method using an aqueous ink suffers from a problem of the deformation of printed paper called curling.
Several methods of reducing or preventing the curling have been proposed.
For example, Patent Document 1 discloses an aqueous ink composition containing an aqueous carrier medium, an colorant, and a curl preventing agent such as sugars and sugar alcohols. Patent Document 2 discloses an ink composition for thermal inkjet printing which contains a low vapor pressure solvent, a water-soluble dye and water and is added with a sugar having a specific structure.
Patent Document 3 discloses an aqueous ink composition containing an aqueous carrier medium, a colorant, and a curl preventing agent selected from carboxylic amides, etc. Patent Document 4 discloses an ink for inkjet printing containing water, a dye, a lower alkyl ether of polyhydric alcohol, and nonionic acetylene glycol, and further containing at lease one compound selected from glycerol and specific polyhydric alcohols.
However, the proposed methods fail to effectively prevent the curling while maintaining sufficient print density; storage stability, and jetting stability.
To improve the printing characteristics, etc., Patent Document 5 proposes to add a monoalkyl ether of polyhydric alcohol such as tetraethylene glycol monoalkyl ethers and a polyhydric alcohol to an aqueous ink containing a water soluble dye or pigment as the colorant.
To improve the print density and print durability, Patent Document 6 discloses an aqueous ink containing an aqueous dispersion of fine polymer particles containing a colorant and a specific polyalkylene oxide derivative.
To improve the print density; gloss and reliability, Patent Document 7 discloses an aqueous ink containing a water-insoluble vinyl polymer constituted by a monomer which is insoluble in water but soluble in a water-soluble organic solvent, a pigment and a water-soluble organic solvent.
To provide resistance to dryness and resistance to friction, Patent Document 8 discloses a pigment dispersion containing a pigment, α-methyl-ω-hydroxypolyethylene glycol ether having an average molecular weight of 250 to 1000 g/mol, and a dispersant.    Patent Document 1: JP 6-157955A    Patent Document 2: JP 6-240189A    Patent Document 3: JP 9-176538A    Patent Document 4: JP 10-130550A    Patent Document 5: JP 9-255904A    Patent Document 6: JP 2001-139849A    Patent Document 7: JP 2006-282779A    Patent Document 8: JP 2004-521182A